The present invention relates generally to the field of acoustic transducers and more particularly to high-volume acoustic transducers. One presently preferred application of the invention is in the field of active noise control, e.g., in a power generation plant.
High-Volume Acoustic Transducers
Sources in which an air flow is modulated can produce very high sound power and sound pressure levels. For example, sirens have been built with acoustic outputs of 26 kW and efficiencies of 30% (sinusoidal) to 70% (square wave). In a siren, the air flow is modulated by one or more chopper valves. Typically, a supply blower, chopping rotor, and motor share a common shaft. A siren is a periodic source and usually can be operated efficiently over only a limited frequency range. Since a siren's mechanical inertia is large, its speed and frequency cannot be changed quickly. Therefore, sirens are unsuitable for use as an active noise control source.
Controllable acoustic sources have been designed using modulating valves. Typically, the valves are driven by a linear motor to modulate the air flow from a supply to an output device such as a duct or horn. These sources can produce acoustic outputs of tens or hundreds of kilowatts. Known designs use comparatively small valves that can be accelerated quickly without requiring prohibitively large mechanical forces. However, such small valves impose a relatively large pressure drop (e.g., 30 psi). Therefore, a large amount of pneumatic power is consumed in driving the air flow through the valves.
Several prior art devices (e.g., Wyle WAS-2000 and Ling EPT 2005) employ a pressure source connected to an output device through a modulating valve whose area is changed with time. The output of this source is superimposed on a steady quiescent flow. These are open-circuit device, requiring a continuous supply of air or other working fluid. Sources such as this are typically operated with their valves half open when the signal amplitude is zero. The input/output characteristics of such sources are markedly non-linear because (1) the valve(s) have non-linear flow characteristics, and (2) there are acoustic non-linearities associated with high-amplitude pressure oscillations. In addition, a large amount of pneumatic power is typically required, and a large amount of flow-generated noise is present even in the absence of a desired acoustic output.
An example of a prior art high output acoustic source is disclosed in Japanese Patent No. 56-116395 (Ueno). The disclosed "speaker with air valve" employs a plurality of valves sized in binary increments. This patent discloses the use of a separate horn for each spatially separated valve. This design is believed to be flawed in several respects. First, time delays between signal components of different amplitude, even those of the same frequency, arise as a consequence of using horns of different sizes. Further, it is difficult to design valves covering a flow range of 2.sup.N for N of about 8 or more while assuring that all valves open in the same period of time. Moreover, the spatial separation of the horns causes the acoustic output to be incoherent, i.e., to bear little relation to the nominal sum of the flows from each valve if operated alone.
One goal of the present invention is to provide a high-volume source for very low frequency applications in which conventional speakers are inadequate. Another goal of the invention is to provide a control mechanism for producing a linear acoustic signal with a simple process that converts the desired acoustic pressure to a sequence of valve operations.
Active Noise Control
As mentioned above, one presently preferred application of the invention disclosed herein is in connection with an active noise control system. Free-field noise sources, such as internal combustion engines and combustion turbines, generate powerful low-frequency noise in the 16 Hz and 31 Hz octave bands (where the 16 Hz octave band extends from 11 Hz to 22 Hz, and the 31 Hz octave band extends from 22 Hz to 44 Hz). Passive noise control requires the use of large, expensive silencers to absorb and block the noise. The size and cost of such silencers makes passive control unacceptable for many applications. An alternative to passive control is a combination of passive control and active control. Passive control abates noise better as the frequency of the noise increases and active control works better as the frequency of the noise decreases. Therefore, a combination of passive and active noise control works best in many applications.
The active control of sound or vibration involves the introduction of a number of controlled "secondary" sources driven such that the field of acoustic waves generated by these sources destructively interferes with the field generated by the "primary" source. The extent to which such destructive interference is possible depends on the geometric arrangement of the primary and secondary sources and on the spectrum of the field produced by the primary source. Considerable cancellation of the primary field can be achieved if the primary and secondary sources are positioned within a half-wavelength of each other at the frequency of interest.
One form of primary field of particular practical importance is that field produced by rotating or reciprocating machines. The waveform of the primary field generated by such machines is nearly periodic and, since it is generally possible to directly observe the action of the machine producing the original disturbance, the fundamental frequency of the excitation is generally known. Therefore, each secondary source can be driven at a harmonic of the fundamental frequency by a controller that adjusts the amplitude and phase of a reference signal and uses the resulting "filtered" reference signal to drive the secondary source. In addition, it is often desirable to make this controller adaptive, since the frequency and/or spatial distribution of the primary field may change with time and the controller must track this change. The present disclosure is directed not to the control algorithm employed in connection with active noise control, but rather to the mechanism employed to produce a high-volume, low-frequency acoustic field.
The strength of an idealized source is measured by the "volume velocity," which is the product of the velocity of a vibrating surface and its area, or the integral of the normal component of velocity over the surface, if the velocity is not uniform. Since conventional loudspeakers have limited displacements, the velocity, and therefore the acoustic output, decreases in proportion to frequency. Conventional loudspeakers are capable of radiating at most a few watts at low frequencies and have efficiencies of a few percent at best. Under these conditions, on the order of one hundred loudspeaker systems may be required for active noise control. Moreover, these loudspeakers require tens of thousands of electrical watts of amplification. Furthermore, conventional loudspeakers are built with minimal cone mass to increase efficiency (which is inversely proportional to the moving cone mass), making such conventional loudspeakers subject to failure when driven at high levels for extended periods of time. This limitation is particularly significant in industrial plant noise control, where the loudspeakers must be placed outdoors in a physically hostile environment.
Another object of the present invention is to overcome the limitations of conventional loudspeakers by providing a compact and rugged high-volume acoustic source capable of producing a high volume velocity output. The high-volume source should be available in a very rugged and yet easily replaceable module, which would make the source easy to maintain.
In addition, another goal of the present invention is to provide a high-volume acoustic source capable of being adapted for use with a variety of fluids. In this regard, there are several instances where fluids other than air would be desired. For example, stack gas may be insonified to produce cancellation of low-frequency or infrasound inside an exhaust stack before it reaches the air; high amplitude signals may be coupled to water for use in sonar systems and undersea communication; and exhaust gas from an engine may be insonified to reduce the pressure fluctuations at the exhaust outlet, reducing the noise radiated from a tail pipe.